The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with imaging head and neck regions with an open MRI scanner and will be described with particular reference thereto. It will be appreciated, however, that the present invention is also applicable to spectroscopy and diagnostic imaging of other regions of the patient and is not limited to the aforementioned applications.
In magnetic resonance imaging, a uniform main magnetic field is created through an examination region in which a subject to be examined is disposed. With open magnetic resonance systems, the main magnetic field is vertical, perpendicular to the subject. A series of radio frequency (RF) pulses and magnetic field gradients are applied to the examination region to excite and manipulate magnetic resonances. Gradient magnetic fields are conventionally applied to encode spatial position and other information in the excited resonance.
Typically, the transmitted RF signals are orders of magnitude larger than the magnetic resonance signals generated by the excited nuclei and detected by the RF receiver coils. To receive the weak resonance signals, localized coils, such as head coils, are often used. Loop or solenoid coils are among the most sensitive coils. However, when two or more are used, they couple strongly which degrades their sensitivity.
With existing structures, it has been difficult to build volume coils consisting of two or more coaxial solenoid coils because of the strong coupling between solenoid coils. When each solenoid element is tuned to resonance frequency, the coupling between the coils detunes the coils causing a separation of the frequencies. Use of additional decoupling circuits typically decreases the SNR of th e coil elements so much that they are unacceptable to be us ed as RF-coils of MRI systems.
U.S. Pat. No. 5,500,596 to Grist and Alley arranges two solenoid coils in such a way that they are sensitive to some of the same imaging area with overlapping loops in the middle of the structure. The overlapping coils are planar coils t hat do not have as high a signal-to-noise ratio as the volume types of coils. The mutual coupling of the top and bottom coils is reduced by positioning the is coils far from each other compared to the coil dimensions.
This further lowers the signal-to-noise ratio of the system.
U.S. Pat. No. 5,804,969 to Lian and Roemer decouples the resonating coils by adding resonant rings. The resonant rings couple to the imaging volume which reduces the Q-values of the main coils. This manifests in lower signal-to-noise characteristics.
The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a diagnostic medical imaging system is provided. A localized RF coil assembly containing four solenoid coils receives resonance signals from excited dipoles in the imaging region. The coils are arranged such that the first and second coils have a common axis perpendicular to an axis common to the third and fourth coils. The coil assembly receives RF resonance signals in quadrature, which are received by one or more receivers.
The coil axis defines the direction of the RF magnetic field for which the coil loop has optimum RF sensitivity within a volume surrounded by the coil loop. A video processor withdraws selected portions of the image representation and converts them into appropriate form for human-readable display.
In accordance with another aspect of the present invention, a method of positioning is provided. Four solenoid coils are arranged in two coil pairs such that the first and second coils, which comprise the first coil pair, have an inherent coupling with each other, and the third and fourth coils, which comprise the second coil pair, have an inherent coupling with each other. In addition, the solenoid coils of the first coil pair have an inherent coupling between the solenoid coils of the second coil pair. Four coil loops, each one connected to a different solenoid coil are positioned adjacent the solenoid coils such that they cancel the inherent coupling between the solenoid coils in the first and second coil pairs and between the first and second coil pair.
One advantage of the present invention resides in an RF-coil structure having a large imaging area.
Another advantage of the present invention is that it allows for coil geometries with mutual inductive coupling.
Another advantage of the present invention is that it allows for a multi-loop RF-coil system.
Another advantage of the present invention is that it provides better image uniformity.
Yet another advantage resides in a higher signal-to-noise ratio.
Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.